The present invention relates to an optical recording/reproducing apparatus for recording and reproducing or recording, reproducing and erasing data by using optical recording medium, such as an optical disk and optical/magnetic disk. More particularly, the present invention relates to an optical recording/reproducing apparatus in which seek and tracking controls are exact and reliable.
A schematic illustration of this type of an optical recording/reproducing apparatus, that is known, is shown in FIG. 1. As shown, the recording/reproducing apparatus is composed of a semiconductor laser (a) as a light source, a collimeter (b) for converting diffusion laser beams emitted from the laser (a) into parallel laser beams, a beam shape prism (c) for reforming a spot shape of a laser beam from an elliptical shape into a substantially circular shape, a beam splitter (e) for splitting an incident laser beam incident on an optical recording medium (d) and a reflecting laser beam reflected from the recording medium (d) one from the other, a 1/4 wavelength plate (f) for converting a laser beam of linear polarization as transmitted through the beam splitter (e) into a laser beam of circular polarization, an objective (g) for focusing laser beams transmitted through the 1/4 wavelength plate (f) on a focal plane of the recording medium (d), and a cylindrical lens (j) for guiding a reflected laser beam emitted form the beam splitter (e) to a quartered PIN photo diode (h) which is for obtaining a tracking error signal, a focus error signal, and a reproducing RF signal.
In the optical recording/reproducing apparatus, to cope with eccentricity of the recording medium (d), and undulation and warp of the same, and the like, the tracking and focus controls are performed by using a tracking error signal and a focus error signal as derived from the quartered PIN photo diode. To read data from the recording medium, a seek control is required to move an optical head from a track to another. To this end, a drive system as described in Japanese Patent Unexamined Publication No. Sho. 63-50927 is assembled into the recording/reproducing apparatus.
The drive system, as shown in FIGS. 2 and 3, is composed of a bobbin (m), a focus adjust movable portion (p), a carriage member (s), and a plate spring (t). A yoke (k) is inserted into the tube of the bobbin (m). A focus coil (fc) and seek coil (sc) are wound around the bobbin (m). The movable focus adjusting member (p) includes an objective (g) mounted on the bobbin (m) by a support member (n). The carriage member (s) carrying the movable focus adjusting member (p) is movable along a rail (q), which is disposed in the radial direction of the recording medium (d), with the aid of a bearing (r). The plate spring (t) is fastened at one end to the movable focus adjusting member (p) and at the other end to the carriage member (s). The plate spring supports the movable focus adjusting member (p), while allowing the portion (p) to move in the direction substantially vertical to the surface of the recording medium (d). When current is fed to the seek coil (sc), an electromagnetic force, or a seek drive force, acts on the seek coil (sc). The seek drive force is transferred through the plate spring (t) to the carriage member (s). By the drive force, the carriage member (s) is moved in the radial direction of the recording medium (d), thereby to effect the seek control and the tracking control. When current is fed to the focus coil (fc), an electromagnetic force, or a focus drive force, acts on the focus coil (fc). By the focus drive force, the movable focus adjusting member (p) is moved in the radial direction of the recording medium (d), thereby to effect the focus control.
Reference character (u) designates a permanent magnet for developing magnetic lines of force directed to the seek coil (sc) and the focus coil (fc). FIG. 4 shows an equivalent mechanism of the drive system in the optical recording/reproducing apparatus as shown in FIGS. 2 and 3. In the figure, (f.sub.s) indicates a seek drive force acting on the movable focus adjusting member (p). (f.sub.f) indicates a focus drive force acting on the movable focus adjusting member (p).
As described above, in the drive system of the conventional optical recording/reproducing apparatus, the electromagnetic force acting on the focus adjust movable porion (p) is transferred through the plate spring (t) to the carriage member (s). By the drive force, the movable focus adjusting member (p), together with the carriage member (s), is moved in the radial direction of the recording medium d), thereby to effect both the seek control and the tracking control. Where a frequency of the current fed to the seek coil is increased to be higher than a present frequency, that is, the movable focus adjusting member (g) and the carriage member (s) are slightly vibrated for the tracking control purposes, the electromagnetic force, which acts on the seek coil (sc) through the plate spring (t), sometimes is not transmitted to the carriage porion (s).
When the electromagnetic force is not transmitted, the simultaneous movement of the movable focus adjusting member (p) and the carriage member (s) is lost and a relative displacement occurs therebetween, to cause a resonance. Under this condition, an unexact tracking control may occur.
FIGS. 5 and 6 are graphs for explaining occurrence of resonance at specific frequencies (several hundreds Hz to several kHz). When the movable focus adjusting member (p) and the carriage member (s) are vibrating, an amplitude of a vibration of the movable focus adjusting member (p) in the focus direction varies against a frequency, as shown in FIG. 5. When the focus adjust movable porion (p) and the carriage member (s) are vibrating, a phase of a vibration of the movable focus adjusting member (p) varies against a frequency, as shown in FIG. 6.